Diagnostic suspicion bias will be avoided by objectively testing all patients who return during follow-up with symptoms or signs suggestive of deep venous thrombosis or pulmonary embolism. Interpretation bias will be avoided by obtaining independent interpretation of follow-up testing in patients with suspected disease by independent adjudicators. The adjudicators will be blinded to the results of the initial ultrasound and the cohort to which the patient belongs. All deaths will be independently reviewed. The adjudicators will be blinded to the cohort to which the patient belongs.